The invention relates to the field of sample preparation for performing chemical assays. More particularly, the invention relates to the field of sample metering valves for extracting a known volume of a sample out of sample containing chamber or a process stream of materials to be assayed.
Typically in chemical processing facilities and chemical analysis labs it is desirable to perform chemical assays. Often this analysis is done by liquid or gas chromatography, although many other methods of assaying samples for their chemical composition also exist. Often, before an assay can be done it is necessary to prepare the sample by isolating a known volume of the sample from a container of same or by isolating a known volume of the sample from a process stream. The portion of the sample so isolated is then diluted by adding the isolated sample volume to a known quantity of diluent to prepare a sample solution of a user defined concentration for the assay.
The amount of sample so isolated must generally be a precisely known volume so that the concentration of the final sample solution can be precisely controlled. There is a prior art valve which has been used to perform this function which is manufactured under the trademark ISOLOK by Bristol Engineering Company of Yorkville, Illinois. This valve uses a T shaped end on a piston within a cylinder. The cylinder is open ended and the piston's T shaped end forms a cap on the cylinder when the piston is in the retracted position. When the piston is the extended position, the end cap of the piston is moved away from the end of the cylinder. The piston has a cylindrical shaped recess therein which is formed a distance up from the distal end of the piston. When the piston is driven to the extended position, the recess is pushed out of the cylinder into the surrounding environment and fills with whatever medium surrounds the valve. The user of the ISOLOK valve takes a sample by causing the piston to extend out into the surrounding environment such that the recess fills with the material which surrounds the valve. Typically, the piston is driven to the extended position by a pneumatic drive arrangement or by a stepper motor. After the recess is filled, the driving apparatus retracts the piston back into the confines of the cylinder. This causes the end cap of the piston to seal the end of the cylinder so that no material outside the walls of the cylinder can get into the cylinder or the recess thereby isolating a known volume of sample material in the recess.
The problem with the ISOLOK valve is in the sealing arrangement. Three O ring seals are used around the circumference of the piston both above and below the recess. These O rings are separated by small spaces, and engage the side walls of the cylinder in sealing engagement. The gaps between the O rings are themselves small recesses, and the gaps around the portion of the piston below the recess are exposed to the surrounding medium when the piston is in the extended position. Because of this fact, the gaps between the O rings around the portion of the piston which is exposed fill with the medium which fills the main sample isolation recess when the piston is put into the extended position. When the piston is retracted into the cylinder, the material trapped between the O rings effectively is part of the isolated sample and is of unknown volume. If the isolated sample is then diluted by extending the piston again into a known volume of diluent, the isolated sample is released into the diluent along with whatever sample is trapped between the O rings. The result that instead of a known concentration of sample in diluent, there is an unknown concentration of sample in the diluent. Further, the inaccuracy of the sample volume is not a constant deviation. There are variations in the error which occur often enough that the predictability of the error is low. This can degrade the precision of the assay.
The ISOLOK valve also is not well suited to dealing effectively with slurries or liquid samples with entrained gas bubbles. The gas bubbles take up volume which could otherwise be filled with liquid and thereby decrease the accuracy of prediction of the exact amount of liquid sample which has been isolated.
Thus a need has arisen for a valve which can accurately and repeatedly isolate a known volume of a sample from a larger volume stored in a container or from a process stream and which can handle the situation of entrained gas bubbles or foam in the sample chamber or process stream gracefully and with precision.